In most cases, the licensing agreement promises very little to the purchaser, and software is supported not out of a legal obligation, but out of a desire for repeat business or to maintain a good reputation.
Open source licenses promise basically nothing. They only place restrictions on the user of the code. Accepting those restrictions is the price you pay for using the code.
However in general you only have an obligation to support your code if you explicitly or at least implicitly promise such support. Most companies will explicitly deny any obligation to support their code because our courts have a habit of upholding the unreasonable expectations of idiots as enforceable obligations.
A lot of proprietary ones don't either.
They only place restrictions on the user of the code. Accepting those restrictions is the price you pay for using the code.
Many proprietary licenses place serious restrictions in addition to having to pay for the software. A lot of vendors even said you couldn't release benchmarks of their products without prior approval from them. Microsoft and Network Associates have invoked this clause to prevent embarassing results from being published. NA's terms were thrown out in court, and Microsoft has gotten a bit better now for Vista at least: you can release results for any .NET benchmarks but only if you follow their rules, which include the release of your complete source code and scripts used to do the benchmark.